The use of feathering blade propellers in sailers, that is propellers with blades capable of orienting themselves by rotating when idle in order to reduce drag when sailing is widespread.
On the other end, in order to optimize overall efficiency when motoring, the propeller should match beside with the characteristics of the engine (torque power and efficiency characteristics in function of the RPM) also with the hydro-dynamic characteristics of the hull and eventually even to prevalent conditions of navigation.
An effective solution to these requirements is provided by the so called variable pitch propellers, that is by propellers wherein it is possible to modify the orientation of the blades in order to adapt within certain limits the pitch of the (screw) propeller to particular characteristics and/or conditions of use.
There is a category of commercially available propellers known as variable pitch propellers with feathering blades that combine both features and are especially used with auxiliary engines in sail boats.
Generally, these propellers comprise a pinion hub that is keyed or in any other way rotated by the drive shaft, on which engage two or more planet gears present at the base of the stems of the propeller blades.
Each blade, the stem of which is pivotally held in a hole of an outer shell, is free to rotate about the axis of the base planet gear upon the run of the planet gear on the conical pinion-hub through two opposite limit angles from a central or neutral position of the blades at which the two major faces of the blades are substantially parallel to the drive shaft under the hydraulic forces acting on it. The limit angles are pre-established by appropriate stops which determine the pitch of the propeller in the two directions of rotation.
The outer shell that is normally formed by sectors joined together by tangential screws, encloses the pinion-hub and the planet gear of the blades that are pivotally sustained in respective holes through the wall of the shell. Of course, the shell is free to rotate about the axis of the pinion-hub within said two opposite limit angles of orientation of the blades as it is either dragged or drags along the blades planetary engaged on the pinion-hub around its axis.
The stops of said arc of freedom of rotation of the shell-blade assembly about the pinion-hub axis are pre-established by the cooperation of circular sectors or radial teeth abutting one against the other and mechanically connected one to the outer shell and the other to the pinion-hub, in order to determine said stops in both directions of mutual rotation of the two parts.
Propellers of this type are described in U.S. Pat. Nos. 4,047,841 and 4,140,434.
In these known propellers, the pitch may be modified only by disassembling the propeller thus requiring the lifting of the boat out of the water whenever the pitch needs to be modified.
In the Italian Patent No. 1,214,251, of the same applicant, a feathering blade propeller was disclosed, the pitch of which could be adjusted without disassembling the propeller.
To this end, a sleeve engageable with the pinion-hub body at different angular positions and held so engaged by a spring is employed.
The stem of said sleeve passing through an axial hole of the ogive terminal of the propeller may be pulled outwardly against the contrasting force exerted by the spring to disengage the sleeve from the pinion-hub and which may then he reengaged in a different angular position on the pinion-hub, thus modifying the pitch.
In subsequent Italian Patents Nos. 1,235,687 and 1,235,831, of the same applicant, further improved propellers of this type were disclosed, according to which it was made possible to adjust the pitch by simply pulling a portion of outer shell in opposition to the force exerted by a contrasting spring, for displacing said part of the shell from the fixed part of the shell by a distance sufficient to disengage a telescopic toothed joint between the two parts of the shell, and to release it after having rotated it by a certain angle to let the displaceable part to engage again with the fixed part of the shell in a desirably modified angular position.
Further advantages of the propellers disclosed in the latter patents mentioned above were provided by an improved manner of mounting the blades that may pivotally turn through the containment and support shell and by the introduction of resilient shock absorbing elements between the abutment surfaces on the end stops in order to dampen the impacts when starting rotation of the propeller in one or in the other direction.
In all these propellers having an adjustable internal kinematic system of transmission of the torque from the drive shaft to the pinion-hub and eventually to the shell-blade assembly of the propeller, it is of paramount importance to dampen the shocks caused by the impacts between the surfaces of the end stops that define the arc of freedom of rotation of the planetary mounted blades, in order to prevent or reduce the audible shock noise when starting to rotate the propeller and that specially when maneuvering for docking or other reasons becomes repetitive because of the numerous inversions of the direction of rotation (forward thrust/backward thrust) as well as to reduce wear and deformation of the metallic stop surfaces of the cooperating parts of the inner torque transmission kinematic system of the propeller.
Even accidental shocks that may be determined by a blade hitting a floating object when motoring may cause violent impacts on the stop surfaces, multiplying the shock noise and eventually cause a dent on the edge of the blade involved with the impact and contribute to the wear and deformation of the internal stop surfaces.
With the objective of enhancing a dampening action of these shocks to which important parts of the propeller are subjected, beside the use of special inserts between the abutting surfaces of the stops that define the angular bounds of freedom of mutual rotation of the shell-blade assembly and of the pinion-hub assembly, additional resilient elements of elastomer have been introduced to share among them part of the stress in order to spread on a relatively large number of elastomer elements the strain and thus reduce the rate of degradation with time of their ability to resiliently absorb the shock stresses and increase the effective lifetime of these resilient shock absorbing elements.
According to a known practice, a typical shock absorbing device for these type of propellers is commonly realized, by purposely realizing the pinion-hub, in two parts.
A first cylindrical flanged sleeve is keyed directly on the drive shaft of the propeller and the flange of the cylindrical sleeve is provided with a plurality of circular cavities, uniformly distributed around a circumference of the flange. An annular insert of elastomer is set inside each of these circular cavities.
The second part of the pinion-hub is constituted by a cylindrical sleeve provided with a terminal flange and that is slid over the cylindrical part of the first sleeve.
This second cylindrical sleeve has at one end a conical pinion toothing while a plurality of pins uniformly distributed around a circumference of its terminal flange and the pins extends from the end face thereof.
Upon assembling the propeller, the second cylindrical sleeve is slid over the first cylindrical sleeve already keyed on the drive shaft, and each annular insert of elastomer set in a respective circular cavity of the flange of the first cylindrical sleeve receives into its central hole the extremity of one of the pins that extend from the terminal flange of the second cylindrical sleeve.
In this way, the accidental shock that may occur to the rotating blades of the propeller as well as the impacts between mutually abutting stop surfaces of the internal kinematic system of the propeller that occur upon starting rotation in a direction, are in part absorbed also by the numerous annular inserts of elastomer that are (pinched) compressed by the respective pins similarly to what happens to the resilient inserts between the metallic stop surfaces of the pitch setting arc of freedom of mutual rotation between the pinion-hub assembly and the outer shell—planetary mounted blade assembly.
In this way, stresses are absorbed by numerous resilient inserts of elastomer, thus proportionally limiting the rate of wear and/or degradation of the elastic properties of the elastomer.
Of course, even if retarded, degradation in time of the shock absorbing ability of these resilient inserts of elastomer cannot be eliminated and therefore these elements must be periodically substituted, and this is normally done during maintenance and antifouling treatments of the hull.
Often, a protracted use of the boat well beyond the times of scheduled maintenance operations for many a reason, bring the propeller to work noisily under conditions of extreme degradation of the resilient inserts of elastomer before substitution of these inserts may take place. This often results in a severe deformation of the metallic stop surfaces which may eventually impose costly repairs and/or substitution of worn out pieces.
A need or opportunity exists of further increasing the lifetime of these shock absorbing resilient inserts of elastomer as well as of preventing that an excessive degradation and/or accidental shocks of extraordinary violence bring about a substantially complete destruction of these inserts and cause intolerable noisy operation of the propeller and a deformation of functionally important metallic stop surfaces.